Field of the Invention
The present invention relates to a protection circuit.
Description of the Related Art
An ultrasound transducer in an ultrasound diagnostic apparatus sends ultrasound waves on the basis of positive and negative high-voltage signals, and outputs a low-voltage signal obtained by receiving these very slight ultrasound waves, to a reception circuit. Therefore, a protection circuit which protects the amplifier circuit that amplifies the low-voltage signals output from the ultrasound transducer, against the high-voltage signals of both negative and positive polarity used to drive the ultrasound transducer, is required between the ultrasound transducer and the reception circuit which send and receive ultrasound waves. When a high-voltage signal is applied, this protection circuit switches off and thereby shuts off the high-voltage signal. Furthermore, when a low-voltage signal is applied, the circuit switches on to enable the low-voltage signal to pass to the reception circuit. For example, whereas a high-voltage signal has a voltage of approximately several ten to several hundred V, of negative or positive polarity, the amplification circuit is composed by devices capable of withstanding a voltage of approximately 5 V.
The number of channels in the transducer of an ultrasound diagnostic apparatus is large (several hundred channels), and the signal transmission/reception circuit is required to have a corresponding number of channels. Moreover, the timings at which transmission pulses are sent in each channel are variable. On the other hand, if the protection circuit is switched on and off using a control signal, then it is necessary to implement continuous control so as to pass low-voltage signals when the circuit is on and to shut off high-voltage signals when the circuit is off, and a number of signal lines corresponding to the number of channels are required and hence costs increase. Therefore, it is necessary to provide a protection circuit having a function for passing low-voltage signals and shutting out high-voltage signals, without the need for an external control signal.
In the prior art, a circuit provided with a circuit for detecting a high-voltage signal, or a circuit employing a depletion-mode FET, has been used in order to achieve a protection circuit that operates without a control signal (see Japanese Patent Application Publication No. 2012-10831 and Japanese Patent Application Publication No. H5-48021).